Organic Chemistry Techniques (Edexcel International A Level Chemistry)

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Reflux, Distillation & Boiling Point Determination

Heating under reflux

  • Organic reactions often occur slowly at room temperature 
  • Therefore, organic reactions can be completed by heating under reflux to produce an organic liquid
  • This allows the mixture to react as fully as possible without the loss of any reactants, products or solvent
    • In distillation, you are trying to separate a chemical or product from a mixture
    • When heating under reflux, you aim to keep all the chemicals inside the reaction vessel

The Heating under Reflux Process:

  • Example reactions where heating under reflux could be used include:
    • The production of a carboxylic acid from a primary alcohol using acidified potassium dichromate
    • The production of an ester from an alcohol and acid in the presence of an acid catalyst
  • The reaction mixture is placed into a pear-shaped or round bottomed flask
  • Anti-bumping granules are, again, added to promote smooth boiling
  • The flask is placed in a heating mantle or it can be immersed in a water bath for heating
  • Quickfit apparatus is then set up with the condenser clamped vertically in place 
    • The joints of the Quickfit apparatus are commonly greased as with distillation
  • A steady and constant stream of water passes through the condenser in a 'water jacket' - it enters at the bottom of the condenser and the drainage pipe removes the water from the top of the condenser
  • The water is heated and the reaction mixture allowed to boil 
  • The heated is stopped and the mixture allowed to cool back to room temperature

Preparation of ethyl ethanoate, downloadable AS & A Level Chemistry revision notes

The preparation of ethyl ethanoate involves heating under reflux for about 15 minutes

Distillation

  • Distillation is a common practical completed in organic chemistry
  • Distillation is used as there are times that a reaction does not go to completion or there are other chemicals produced as well as the desired product
  • Distillation allows you to separate compounds by their boiling point
    • Chemicals with the lowest boiling point will distill first
  • One of the most common distillation practicals is the oxidation of primary and secondary alcohol to aldehydes and ketones

The Distillation Process:

  • To produce an aldehyde from a primary alcohol, a reaction mixture containing the primary alcohol and acidified potassium dichromate solution is placed into a pear-shaped or round bottomed flask
  • Anti-bumping granules are added to promote smooth boiling
  • Quickfit apparatus is then set up, including a still head and condenser connected to the side
    • The joints of the Quickfit apparatus are often have a thin layer of silicon grease smeared over them to give a better seal as well as to make it easier to disassemble the equipment afterwards
  • A Quickfit thermometer can be used, with the thermometer bulb sitting where the vapours will pass into the condenser
  • A steady and constant stream of water passes through the condenser in a 'water jacket' - it enters at the bottom of the condenser and the drainage pipe removes the water from the top of the condenser

Carbonyl Compounds Synthesis of Aldehydes, downloadable AS & A Level Chemistry revision notes

Heating under Distillation Apparatus

  • The reaction mixture is heated until it boils using a heating mantle
    • Electric heating mantles are used for this because the temperature can be controlled, and because you are using chemicals which are flammable
  • The distillate which forms in the condenser drips directly into a receiving vessel
    • The distillate which should be collected, is that which is given off at +/- 2 oC of the boiling point of the desired product
    • Some distillate may be given off below this temperature - this needs to be discarded and a clean vessel used to collect the desired product
    • Stop collecting the distillate if the temperature rises above +/- 2oC of the boiling point of the desired product

  • The aldehyde product has a lower boiling point than the alcohol (since it has lost the H-bonding) so it can be distilled off as soon as it forms

Examiner Tip

  • These practicals give you the opportunity to discuss:
    • The use of an electric heating mantles and water baths rather than a Bunsen burner 
    • The choice and setup of laboratory apparatus 
    • Health and safety considerations including the careful handling of different liquids, including those which are corrosive, irritant, flammable and toxic

Boiling point determination

  • The boiling point of a liquid is indicative of its purity and identity
  • Boiling point is determined by distillation
  • The sample is gently heated until it boils and this temperature is recorded
    • This boiling point can then be compared against literature / database values
  • If the sample contains impurities:
    • The boiling point may appear higher than the literature / database values
    • The sample may boil over a range of temperatures instead of at a single temperature

Solvent Extraction & Drying

  • There are different methods to purify organic compounds, including:
    • Solvent extraction / use of a separating funnel
    • Use of drying agents 

Solvent extraction / use of a Separating Funnel

  • When organic liquids are being prepared, water can often be obtained along with the organic product
  • The water will usually form an aqueous layer with the product in the organic layer
    • It can sometimes be hard to identify which layer is the organic layer - this can be achieved by simply adding water and seeing which layer increases in volume
  • Other organic reactions may need to be neutralised before being purified, this can be achieved by adding sodium carbonate solution to the reaction vessel or separating funnel
    • This can also be used to remove other impurities by washing

  • The contents of the reaction vessel are transferred to a separating funnel and a stopper added
  • The separating funnel is inverted and the stopcock opened to release the pressure - this is repeated 15-20 times
    • If neutralisation has occurred then the stopcock is opened slowly to avoid losing any product
  • The two layers are allowed to separate 
    • In the following example, the aqueous layer is the bottom layer inside the separating funnel
  • The stop cock is opened so that the aqueous layer drains away and the organic layer can be drained into a clean beaker

Separation of ethyl ethanoate, downloadable AS & A Level Chemistry revision notes

A separating funnel allows the product to be cleaned and isolated

Use of drying agents

  • Drying agents can be used to remove traces of water from an organic product
  • Drying agents are usually anhydrous inorganic salts that readily hydrate in the presence of water
    • Anhydrous calcium chloride is commonly used to dry hydrocarbons
    • Anhydrous calcium or magnesium sulfate are used as more general purpose drying agents
  • A spatula of drying agent is added into the organic product and swirled
    • If the organic product has a low boiling point, a lid / stopped can be added to reduce the potential evaporation of any product
  • If the drying agent clumps together, then there is still water in the organic liquid
  • More drying agent is added until some remains dispersed in the organic liquid as a fine powder
  • The dry organic liquid can then be decanted or filtered
    • If the organic liquid is dry then it should appear clear

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.